USGS-474-138, 'Seismicity of the Pahute Mesa Area, Nevada ...

HAMILTON

USGS-474-138 USGS-474-138

UNITED STATESDEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

Nationa1 Center for arthquake Research345 Middlefield Road

Menlo Park, California 94025

SEISMICITY OF THE PAHUTE MESA AREA,NEVADA TEST SITE

8 December 1968 through 31 December 1970

(SPECIAL SUDIES-89)Date publised: 1971

Prepared underAgreement No. AT(29-2)-474

for the

Nevada Operations OfficeU.S. Atomic Energy Commission

BLANK PAGE

{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

BLANK PAGE

SPECIAL STUDIES-891971

UNITED STATESDEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

National Center for Earthquake Research345 Middlefield Road

Menlo Prk, California 54025

SEISMICITY OF TE PAHUTE MESA AREA,NEVADA TEST SITE


by

R.M. Hamilton, b.E. Smith, F.C. Fischer, and P.J. Papanek{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

ILLUSTRATIONS

Page

Figure l.--Map of the region surrounding Pahute

2 --Map of the Pshute Mess area showing locations

of the underground nuclear explosions dis-

cussed in this report-------------------------- 4

3.--Locations of telemetered seismograph stations

in operation on 31 December 1970--------------

4.--Locations of telemetered and portable seismo-

graph stations n operation for the Benham

explosion -------------

5.--Locations of telemetered and portable seismograph

stations in operation for the Jorum explosion

6 --Locations of telemetered and portable seismo-

graph stations in operation for the Handley

explosion- - ----- 15

7.--Block diagram of the seismic telemetry system--- 17

8.--Response of the seismic telemetry system-- ----

9. Rate of earthquake occurrence from 8 December 1968

10 - l8.--Maps of epicenters-----------------------------

19.--Depth distribution of earthquakes- -------------

20.--Epicentral distance distribution of earthquakes-- 52

21.--Frequency-magnitude distribution--- 54

Page

Figure 22.--Map of all epicenters -------------- 56

23.--Fault-plane solutions for aftershocks of Benham,

Jorum and Handley plotted on a geologic map 59

24.--Epicenter map of earthquakes with relatively

well-determined focal depth---------------- 62

25.--Vertical sections------------------------------ 63

TABLES

Table l.--Telemetered-seismograph data 8

2.--Portable-seismograph data for Benham------------- 10

3.--Portable-seismograph data for Jorum--------------- 12

4.--Portable-seismograph data for Handley------------- 14

5.--Velocity model used in locating seismic events---- 21

UNITED STATESSPECIAL STUDIES-89 DEPARTMENT OF THE INTERIOR USGS-474-138

GEOLOGICAL SURVEY

National Center for Earthquake Research345 Middlefield Road

Menlo Park, California 94025

SEISMICITY OF THE PAHUTE MESA AREA,NEVADA TEST SITE


by

R.M. Hamilton, B.E. Smith, .G. Fischer, and P.J. Papanek

ABSTRACT

The underground nuclear explosions Benham, Purse, Jorum andHandley, detonated on Pahute esa, initiated earthquake sequenceslasting approximately 70, 10, 20 and 60 days, respectively. Earth-quakes of magnitude 2.0 or larger in these sequences numbered 2012,24, 159 and 231, respectively; earthquake magnitudes were all lessthan 5. The explosion Pipkin, also detonated on Pahute Mesa, hadno apparent effect on seismicity. Ninety-four percent of te earth-quakes with well-determined focal depths occurred shallower than5 km, and 95% of the located earthquakes were within 14 km of groundzero of the preceding explosion. There is no evidence for explosion-stimulated earthquake activity extending outside the area of PahuteMesa. The patial distribution of earthquakes appears to be largelycontrolled by geologic structure; however, the epicenter distributioncan be associated wish observed fault movement only for aftershocksof Handley. Fault-plane solutions indicate predominant dip-slipmovement in the northern part of the Pahute Mesa area for aftershocksof Benham, Jorum and Handley. In the southern part, dextral strike-slip movement was found for aftershocks of Benham and Hardley. Thefrequency-magnitude relationships are similar for earthquakes follow-ing Benham, Purse, Jorum, and Handley.

BLANK PAGE

INTRODUCTION

Seismic activity in the Pahute Mesa area of the Nevada Test Site

(NTS) has been monitored continuously by the National Center for Earth-

quake Research, U.S. Geological Survey, since December 1968. The main

purposes of this program are to obtain an accurate description of the

earthquake activity in the area, to establish its relationship to under-

ground nuclear explosions and geologic structure, and to develop an

understanding of the mechanism by which the earthquake activity is

stimulated.

The Pahute Mesa area (Figure 1), as referred to in this report,

extends westerly from the western margin of Yucca Flat to beyond Black

Mountain, and northerly from south of Timber Mountain to include the

southern part of Gold Flat and the Kawich Range. This area was chosen

for discussion because it includes the sites of the high-yield tests

at NTS as well as the areas seismically affected by these tests.

This report covers the seismic monitoring program to 31 December

1970. Up to that time, six explosions at Pahute Mesa had been monitored.

In addition, during this period there were six explosions at Rainier

Mesa and one explosion at Mine Mountain, both areas located in the

southeastern part of the Pahute Mesa area. Relevant data concerning

the explosions are listed in the Appendix, and their locations are

plotted on a map of the Pahute Mesa area in Figure 2.

Of the explosions listed in the Appendix, the Benham test has

received the greatest attention with regard to its effect on seismicity

Figure l.--Map of the region surrounding Pahute Mesa. The areaof Figures 2 and 10-18 is outlined.

3


Figure 2. Map of the Pahute Mesa area shoving locations of theunderground nuclear explosions discussed in this report. Someboundaries of NTS are shown for referen .e. Symbols and theexplosions they represent are, respectively: S - Schooner,B - Benham, W - Wineskin, C - Cypress, P - Purse, J - Jorum,K - Pipkin, D - Diesel Train, A - Diana Mist, H - Handley,L - Mint Leaf, U - Diamond Dust, and N - Hudson Moon.

(Hamilton, McKeown and Healy, 1969; Hamilton and Healy, 1969; Healy,

Hamilton and Raleigh, 1970; Stauder, 1971). Data concerning the earth-

quakes caused by Benham were presented in a report covering the seismic

monitoring to 30 June 1969 (Hamilton and others, 1969). Since the report

was issued, additional data have been obtained for the period it covered.

Approximately 150 hypocenters were determined in three short intervals

of the recording period for a detailed study by Stauder (1971). Also,

magnitudes have been assigned to the located earthquakes, and the

approximate origin time and magnitude have been determined for many

other shocks. The earlier data are repeated in this report, with a few

corrections; thus this report provides a complete data summary and

supersedes the earlier one.

A number of organizations have an interest in the Pahute Mesa

seismic activity, and have made use of the data gathered by the U.S.

Geological Survey seismic network. It is hoped that this report will

facilitate further use of the data.

5

BACKGROUND

The seismic studies of the U.S. Geological Survey at the Nevada

Test Site were prompted by reports of important earthquake activity

following the Boxcar event on 26 April 1968 (Ryall and Savage, 1969),

and by similar reports for some earlier events (Boucher and others,

1969). The existing regional seismic network in southern Nevada was

insufficient to yield accurate hypocenter determinations for this

activity, with the result that although most of the earthquakes

probably occurred near ground zero, uncertainty in their location was

tens-of-kilometers. This uncertainty was lower for Boxcar aftershocks,

which were monitored with a small tripartite array (Ryal and Savage,

1969). The seismic program of the U.S. Geological Survey was designed

to reduce these hypocenter uncertainties by employing the techniques

of dense seismograph networks developed for studying the San Andreas

fault zone in central California. The decision to undertake such a

program was made shortly before the Benham test of 19 December 1968.

For this test, seven telemetered seismic stations and 20 portable

seismographs were installed. The seven telemetered stations were

placed at the only sites where telephone service could be obtained.

These sites were generally on the southeast side of Pahute Mesa, and

by themselves provided poor coverage. In June 1969, therefore, the

network was upgraded by installing seven additional stations using

radio telemetry. Radios were also installed at three of te original

seven sites because telephone service was discontinued. The total of

6

14 stations was still in operation at the end of 1970 (Table 1 and

Figure 3).

The portable seismographs installed for the Benham test were

operated until about one month after the explosion (Table 2 and Figure 4).

These units were installed again for a two-week recording period after

the event (Table 3 and Figure 5), and for 4 weeks after the

Handley event (Table 4 and Figure 6). Benham, and Hanlley, each

with yield of about one megaton were the largest explosions at NTS

during the period covered by this report.

Table .- Telemetered-Seismograph Data{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

Figure 3.-Locations f telmetered seismograph stations in operationon 31, December 1970. Station data are given in Table 1.

Table 2.-Portable-Seismograph Data for Benham{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

Figure 4.--Locations of telemetered and portable seismographstations in operation for the Benham explosion. Station

data are given in Tables I and 2.

Table 3.--Portable-Seismograph Data for Jorum

Code Lat. N. Long. W Correction* Recording Period

01 370 32.91'

02 25.46'

03 10.89'

04 03.03'

05 15.44'

06 30.42'

07 21.27'

08 17.68'

09 1'.86'

10 19. 19'

1160 19.64'

09.23'

10.39'

22.96'

47. 30'

40.70'

25.57'

24.21'

29.34'

29.58'

-0.70 sec

-0.21

-0.49

-0.36

-0.40

-0.58

-0.17

-0.06

0.02

0.04

09/13/69

09/13/69

09/14/69

09/14/69

09/13/69

09/13/69

09/14/69

09/14/69

09/14/69

09/14/69

- 09/21/69

- 09/21/69

- 09/23/69

- 09/23/69

- 09/21/69

- 09/21/69

- 09/21/69

- 09/24/69

- 09/24/69

- 09/24/69

*Subtracted from seismic wave arrival time.

12


Figure 5.--Locations of telemetered and portable seismographstations in operation for the Jorum explosion. Station

data are given in Tables I and 3.

13

Table 4 .-- Portable-Seismograph Data for Handley

Code Lat. N. Long. W Correction* Recording Period

01

02

03

04

05

06

07

08

09

10

11

37 17.82'

31.43'

32.90'

16.99'

3.67'

16.93'

19.72'

19. 78'

18.33'

15.48'

20.99'

116 51.21'

43.21'

19.71'

12.99'

26.76'

34.46'

33.78'

31.21'

27.67'

30.45'

28.08'

-0.63 sec

-0.58

-0.64

-0.09

-0.45

-0.16

-0.14

0.0

0.1

0.0

0.0

03/17/70

03/15/ 70

03/15/70

03/13/70

03/16/70

03/16/70

03/16/70

03/16/70

03/16/70

03/16/70

03/22/70

- 04/11/70

- 04/11/70

- 04/11/70

- 04/09/70

- 04/09/70

- 04/11/70

- 04/10/70

- 04/10/70

- 04/10/70

- 04/10/70

- 04/11/70

*Subtracted from seismic wave arrival time.

14

SEISMIC INSTRUMENTATION

The telemetry system can be explained by a block diagram

(Figure 7). The equipment at each station includes a seismometer

(Mark Products L-4C, or Electrotech EV-17, vertical component, 1 Hz),

a package containing a preamplifier and voltage-controlled oscilla-

tor (Develco, Model 6202), and batteries. The frequency-modulated

tone produced by each section is carried by wire or by radio to a

central point where it is combined with tones from six other stations.

The resulting multiplexed signal is then transmitted by voice-grade

telephone circuits to the Geological Survey office in Menlo Park,

California. At that point, the multiplexed signal is recorded on

magnetic tape, and the seven channels of data on each telephone line

are separated and demodulated by discriminators (Develco, Model 6203),

and recorded on 16-mm film (Geotech Division, Teledyne Industries,

Develocorder). Readings from the film yield locations and magnitude

determinations of the earthquakes.

The overall response of the seismic system is plotted in

Figure 8. The magnification data given are for a system adjusted to

produce a 1-mm peak-to-trough record amplitude for a 10 v rms, Hz,

calibration signal, input in place of the seismometer. The true

magification of each station is obtained by multiplying the given

magnification values by the actual record amplitude of the calibra-

tion signal. These amplitudes are usually near 10 mm, which

corresponds to a peak magnification of 3.3 x 106.

16


Figure 7--Block diagram of the seismic telemetry system.

17

Figure 8.--Response of the seismic telemetry system.

The portable seismographs are described in detail by Eaton and

others (1970).

HYPOCENTER DETERMINATION

Hypocentezs re computed from arrival times of P seismic waves

using the program HYPOLAYR (Eaton, 1969). The odel of seismic

velocities used in the earthquake location program (Table 5) was

derived from three sources: a velocity log obtained in the

deep exploration hole U20f (R.D. Carroll, written communication).

a summary of Pahute seismic characteristies (F.A. McKeown,

written communication). and a synthesis of the U.S. Geological

Survey long distance refraction profiles in the Nevada Test Site

region (Prodehl. 1970). Allowance for local departures from this

model was made by applying station corrections computed from readings

of the explosions.

Table 5.--Velocity Model Used in Locating Seismic Events{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

EARTHQUAKE MAGNITUDES

The magnitudes listed for the earthquakes were determined using

the duration of the seismic signal. Signal duration as a measure of

magnitude has previously been used by Bisztricsany (1958) and Tsumura

(1967). A basis for use of the technique for local earthquakes is

given by Aki (199). He pointed out that duration is virtually in-

dependent of epicentral distance within 100 k, and suggested that the

energy in the coda of signals from local earthquakes comes from back-

scattered waves. It is an important advantage of the duration

technique that it is not strongly distance-dependent, hence magnitudes

can be estimated for events only approximately located. Another

advantage is that the technique can treat earthquakes over a wide

magnitude range because signal clipping is not a factor.

In this study, signal duration was measured from the average onset

time at the Pahute Mesa seismic network to the average time when the

peak-to-trough amplitude dropped below 1 cm for the last time. An

amplitude of 1 cm was chosen because it is above usual noise conditions.

Stations are generally adjusted with a background noise level of

several mm record amplitude. The duration technique for estimating

magnitude partly compensates for the effects of local lithology:

stations on poorly consolidated material often record higher ground

motion, but are set with a lower instrumental gain because they also

see a higher level of background noise.

To convert signal duration to magnitude, a relationship developed

22

for the U.S. Geological Survey seismic network near Rangely, Colorado,

was adopted (J. H. Healy, written communication). This relationship

was derived with dta ranging in magnitude from -0.5 to 2.5:

M - 1.824 Log 1 D - 1.036 (1)

where MD is the magnitude and D is the signal duration in seconds.

M is supposed to be equivalent to Richter's local magnitude, ML.

The validity of equation (1) for the NTS region was checked for

earthquakes below about magnitude 2 by computing ML following the pro-

cedure of Eaton and others (970) using recor from the portable

seismographs. Values generally agreed within a few tenths. At the

higher magnitudes (up to 4), values obtained using equation (1)

agree well with ML values derived from Mb reported by the U.S. Coast

and Geodetic Survey (USCGS) using the equation relating ML and

given by Richter 1958, p. 366). The existence of systematic dis-

crepancies in m values has been shown by Evernden (1967). Basham

and others (1970) computed magnitudes for 43 aftershocks of Benham

using Evernden's regional formulas. These magnitudes on the average

are about 0.7 units lower than the USCGS values, but agree closely

with magnitudes computed using equation (1): on the average the

regional magnitudes reported by Bsham and others are 0.11 + 0.29

larger than the signal duration magnitudes. in short, the magnitudes

assigned in this report are intended to correspond with ML, but in

addition agree well with based on regional formulas.

23

DATA SUMMARY

A chronological listing of the announced underground nuclear

explosions and seismic activity is given in the Appendix. All hypo-

center determinations are included. In addition, the approximate

origin time is given for many events that were not located. Known

chemical explosions were eliminated from the list.

Many factors prevent the list of seismic activity from eing

uniform. The nature of the activity introduces many difficulties.

Following an explosion and continuing until cavity collapse, the signals

are almost constantly off-scale or clipped. Some discrete seismic

events can be identified, but many events overlap. Smaller events

during this period would be obscured. This high level of activity is

presumably associated with cavity deterioration, as indicated by its

cessation with cavity collapse and its location in the cavity vicinity,

although it is difficult to locate most of the events reliably. Many

of the cavity-related events are characterized by very emergent onsets

and relatively low-frequency motion, about 1 Hz. Mixed with this signal

are seismic events that have the appearance of local earthquakes. They

have sharp osets and motion predominantly near 10 Hz. Most of these

events are located away from the cavity.

Another reason for non-uniformity of the data is the variation in

instrumentation. Near the time of the Benham, Jorum, and Handley

explosions, approximately 30 seismographs were deployed within about

30 km of the explosion sites. In contrast, from January through June

1969 seven telemetry stations were the only units in operation.

24

A further complication in the earthquake list is that the records

were not processed in a consistent manner over the whole reporting

period, primarily because of the very high level of earthquake activity

caused by the Benham explosion.

The earthquakes listed from 8 December 1968 to the Benham detona-

tion, at 163OZ on 19 December, are all those that could be located.

From Benham to 1700Z on 19 December the list includes those earthquakes

that could be recognized as discrete events in the overall high level

of activity. Recognition of events in this period of highest activity

was based partly on records from a telemetered station that was

affected by the explosion in such a way as to greatly reduce its

magnification. Subsequent inspection of the seismometer revealed that

the mass was "hung up."

From 1700Z on 19 December to 0700Z on 24 December the list is

thought to include all recognizable earthquakes of magnitude 2.0 or

larger. In this period, earthquakes were selected for location to

sample important episodes of activity, and to include shocks in

areas. Three six-hour intervals were studied in detail by Stauder

(1971): 0500-l1OOZ and 1200-1800Z on 20 December, are 1000-1600Z on

22 December.

From 0700Z on 24 December to 2400Z on 31 January 1969 earthquakes

of magnitude 1.4 or larger are listed. Again, selected shocks were

located.

After 31 January 1969 the policy in processing the records was

to locate all earthquakes that could be located, and to note the

approximate origin time of other shocks of magnitude 1.4 or larger.

Many of the earthquakes located had a magnitude below 1.4.

Undoubtedly, there are some omissions from the list. Approxi-

mately 15 minutes of record is lost each day when the film is changed;

usually this is done at about OOOOZ. Occasionally the Develocorder

malfunctions, and sometimes telephone service is lost. tn such a

large volume of data, there are certainly mistakes in processing.

Taking all these problems into consideration, it is felt that errors

and omissions could have only a minor effect on the ypes of analysis

included in this report.

TEMPORAL VARIATIONS IN SEISMICITY

The variation in the level of earthquake activity with time is

shown in Figure 9. The number of earthquakes n each two-day period

was obtained from the list in the Appendix, separate counts being

shown for all shocks in the list and for shocks of magnitude 2.0 or

larger. Because of the way which the earthquake list was formed,

the data for the larger shocks are more suitable for showing the time

variation n seismicity over the hole monitoring period. The count

of all earthquakes gives a good indication of the level of activity

except immediately following the larger explosions, Benham, Jorum,

and Handley, when the level would be underestimated.

The three main periods of scismic activity followed the three

megaton-yield explosions Benham, Jorum, and Handley. Of these the

Benham aftershock sequence was longest, Handley's was intermediate

and Jorm's was smallest. interesting variations in the rate of

earthquake occurrence exist in each of these sequences. The Purse

test also is associated with an increase n activity. Other peaks

in activity exist that are not clearly associated with an explosion.

The explosions in the Rainier Mesa area had no apparent effect on the

seismicity.

In discussing the temporal variations in the earthquake activity

in detail, it will be helpful to consider at the same time the

associated spatial variations. The epicenters listed n the Appendix

are plotted in Figures 10-18. The time periods covered by the

8 DEC 1968 TO BENHAM

Figure 10

BENHAM THRU 31 DEC 1968{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}JORUM THRU 31 OCT 1969

1 NOV 1969 TO HANDLEY{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

HANDLEY THRU 30 APR 1970{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

1 MAY 1970 THRU 31 DEC 1970

epicenter maps were chosen to display the effects of the various

explosions and important other earthquake activity. All epicenters

listed plotted, regardless of quality. Plots were also prepared

of only the better located earthquakes; however, these were not used

because they do show important ctivity that occurred when the

seismic network was relatively poor or that was situated in an area

not well covered by the network. Also, inclusion of poorly located

earthquakes does not obscure the general epicenter pattern. On each

of the epicenter maps some boundaries of the MTS and the sites

of the explosions on Fahute Mesa are plotted or reference. Explosions

before the end of the period covered by each figure are represented

by solid triangles, whereas those not yet detonated are represented

by open triangles.

Figure 10 - December 1968 to on 19 December 1968):

In the 12 days of monitoring preceding seismic events

were Seven of these occurred in the zone that was later

strongly activated by Benham.

The Schooner explosion on December apparently had no effect

on the seismicity. Schooner was a excavation experiment

with a device burial depth of Springer and Kinnamann, 1971).

In earlier publications (Hamilton and Healy, 969; Hamilton,

McKeown and Hesly, 1969) two seismic events located k southeast

of Schooner one 32 before Schooner and the other

After it, were reported as earthquakes. It hs been learned that

Although most of the earthquakes during this episode of increased

activity occurred in the previously active zone, some of the shocks

occurred as part of a southerly extension of the epicenter lineation

that runs to the nuth from a point 3 km west of Bnham. Before

commencement of the episode the most southerly epicenter in tnis

lineation had a latitude of 37' 10.6'. On 6 January this limit was

extended by a small amount to and on 11 January an earth-

quake occurred as far outh as 37 8.8'.

This episode o increased rate of earthquake occurrence and the

associated growth of the seimically active zone is probably the most

important tectonic event in the period covered by this report. It

is also the one most n need of further study, as at the present

time epicenters have been determined for only a small percentage of

the locatable shocks. The seismograms were, however, examined for

shocks in unusual places, as estimated from relative arrival times,

so it is felt that the general nature of the activity is correctly

represented.

In terms of seismic energy release the 6 to 13 January 1969 earth-

quake sequence rnks high compared with the rest f the reporting

period. During the sequence 34 shocks with 3.0 were recorded.

Prior to the sequence there had been 70 Benham aftershocks in this

magnitude range; however, any of these resulted from cavity

deterioration, and in ny case did not approach in number the

activity in the 6 to 13 January sequence. Following the sequence,

the rate earthquake occurrence generally declined until the Purse

detonation.

42

Superimposed on the general decline in activity following Benham

are peaks in activity that curiously suggest a periodicity of about

20 days (indicated by the small arows in Figure 9). The data for

and to some extent the data for all shocks show six such

peaks. In each case the activity is higher than that of the previous

few days. A cause for such periodicity does not immediately come to

mind. Correlation of the periodicity with natural processes would

require an element of coincidence, since the series of peaks starts

at the time of Benham. It is difficult to imagine an effect of the

explosion that could have such a long period and that could continue

to manifest itself for approximately four months.

The epicenter distribution of Figure 12 covers most of the area

covered by the activity shown in Figure 11. The northeast-striking

zone passing 3 km north of Benham in Figure 11 is not as active in

Figure 12. As discussed above, new activity resulted in a southerly

extension by over 3 km of the zone of activity 7 to 10 km south-

southwest of Benham. Other new activity was centered about 10 km

west-southwest of Benham, on the west side of the main north-striking

epicenter lineation. In Figure 11 activity east of this lineaction

was seen, and taken together the two groups of activity define an

east-striking epicenter zone. Another east-striking zone s in-

dicated about 5 km further to the south.

Figure 13 - Purse thru 30 June 1969:

Detonation of the Purse explosion reactivated the seismic zone that

was activated by Benham. In the last days before Purse, six

43

earthquakes are listed. In the first days after the test, 3

earthquakes are listed. These earthquakes occurred in two episodes,

as indicated by the two peaks in Figure 9. The first episode started

with the shot and lasted through 9 May with the activity being almost

entirely in the northern part of the activated region, within 5 km of

Purse Then the second episode began gradually with most of the

Activity coming on 14 and 15 ay in the southern part of the zone

about 6 to 9 km from the explosion. The episodic nature of the post-

Purse activity was also seen in the post-Benham activity.

A cluster of epicenters is shown in Figure 13 bout 4 east of

Purse and 4 km south of Jorum. The location of this activity was,

with the exception of a few shocks, at the northeastern end of one of

the main Benham aftershock zones. Later it was the starting point

for a northeasterly extension of the seismically active zone for Jorum.

Figure 14 - 1 July 1969 to Jorurm. on 16 September 1969)

This period was chosen to display the earthquake activity 7-10 km

southeast of Benham The activity mostly lies in two epicenter

clusters, but altogether it defines a zone striking northeasterly

that subparallels the epicenter zones that lie northwest of Benham

(Figure 11). This particular area had not previously been active

The activity in the trend started in the southern cluster with a

sequence of events on July, followed by another sequence starting on

12 July Earthquakes occurred sporadically in this area until 8 August

when a magnitude 3.0 earthquake led off the activity in the northern

44

cluster. Activity in this area continued into early September.

The activity southeast of Benham is interesting for several reasons.

The northeasterly strike of the zone suggests that the activity is

associated with the same fault system delineated by the activity north-

west of Benham. The fact that te activity occurred more than six months

alter Benham and two months after Purse, in an area not previously the

site of activity, raises the possibility that it was not caused by test-

ing, but instead is natural. If this is indeed the case, then the

similarity in strike of the shot-induced and natural epicenter zones

provides further evidence that the explosions stimulate natural earth-

quake processes. If, on the other hand, we assume that the activity was

caused by the explosions, which is thought to be the more reasonable

assumption since the earthquake activity in the region urrounding NTS

is so low, the elayed nature o the activity may mean that the

effects of an explosion may not manifest themselves until months after

detonation.

Activity continued in the seismic areas affected by Benham and Purse.

Jorum through 31 October 1969:

As in the case of Benham, the Jorum explosion initiated an intense after-

shock sequence. The duration and number of shocks in the sequence,

however, were much lower than in the Banham sequence (Figure 9). Follow-

ing Jorum, the number of earthquakes with M > 2.0 in a two-day interval

dropped to less than after six days, whereas 46 days were required to

reach the same level following Benham. A very low level of activity was

reached by mid-October, followed by a resurgence of activity that peaked

on 28 October (Figure 9).

45

In the first four hours after detonation of the Jorum test, earth-

quake activity was mostly located in the cluster that surrounds and

covers ground zero. During this period all 15 Jorum aftershocks with

with M > 3.0 occurred, the last one at 1815Z probably being associated

with final cavity collapse. The 13 of these larger shocks that could

be located were within 1 km of the shot. Later, activity began in te

cluster that was stimulated east of the Purse explosion (Figure 13), and

the northeasterly-striking extension of the Benham aftershock zone

developed.

The Pipkin test at 1430Z on 8 October apparently had little effect

on the seismic activity. Five epicenters were determined near ground

zero, where previously only one event had been located. That Pipkin was

detonated 85 ft. above the water table, whereas the other Pahute Mesa

explosions were below tne water table (Springer and Kinnamann, 1971) may

be an important observation in understanding the cause of explosion-induced

earthquakes (Healy and others, 1970).

The small arrows in Figure 9 at 20-day intervals after Jorum were

plotted to check for the existence of a periodicity similar to that

suggested in the earthquake rate following Benham. There is no peak

corresponding with the first arrow after orum; however, the second and

third arrows correspond with peaks.

Figure 16 - 1 November 1969 to Handley (1900Z on 26 March 1970):

During this 146-day period, 141 earthquakes arc listed in the

Appendix for a rate cf about one a day. The epicenter distribution

was fairly diffuse over much of the area that had previously been

active (Figure 15). Activity continued in the vicinity of Jorum,

46

though at a low level. The easterly trend passing 4 km south of

Benham was active, particularly at its western end in an area later

affected by Handley.

Figure 17 - Handley through 30 April 1970:

The Handley explosion caused an aftershock sequence intermediate

in level between that of Benham and Jorum (Figure 9). Again, the

seismic activity commenced at a high rate following detonation. It

then declined fairly steadily through April and into erly May.

The seismic activity during the first 24 hours after Handley,

although at a high level, was almost cpletely associated with

cavity, judging rom its character. Surface collapse occurred 23 hr

42 mn after detonation (Springer and Kinnamann, 1971). Tectonic

earthquake activity began relatively slowly, with only six locatable

shocks being detected in this period despite a special effort to find

such events. The rate of occurrence non-cavity activity increased

considerably early on 28 arch, two days after Handley.

Most o the Handley aftershocks occurred in a circular area

from Handley to about 4 km to the west. Activity in the area about

12 km south-southwest of Handley began slowly: The first located

earthquake there was at 1124Z on 28 March, the second at

March, and the third at 1021Z on 30 March; ost of the shocks

occurred on 4, 10, and 11 April.

The three eicenters about 30 k southeast of Handley lie at

the southeastern end of a trend that s revealed on the next man.

47

Figure 18 - 1 May trough 31 December 1970:

During this period the Hndley aftershock sequence continued to

decay, with a resurgence in the middle of May (Figure 9). By the end

of May, activity had dropped below a rate of two earthquakes a

day. From 1 June to the end of 1970, 202 earthquakes were counted,

a rate of about one a aay.

Activity continued in the area west of Handley where the main

activity occurred earlier. The area 1 km south-southwest of Handley

also remained active.

About 30 km southeast of Handley a northeast trend of activity

gradually developed. Earlier (Figure 17), three epicenters were

determined at the southwestern end of this trend.

There is no suggestion of the 20-day periodicity in earthquake

rate following Handley (Figure 9).

48

GENERAL CHARACTERISTICS OF TE EARTHQUAKE DSTRIBUTION

The earthquakes listed in the Appendix with a quality rating

of A or B. which means that focal depth is relatively well determined,

were selected to examine the overall depth distribution of activity.

A histogram for these shocks (Figure 19a) shows a peak for the depth

range 3 to 4 km. A sharp decline in activity occurs below 5 km, with

94% of the 1480 earthquakes represented in the histogram shallower

than that depth. Only six hypocenters were computed to lie below

8 km; in view of the uncertainties involved in determining hypocenters,

it is possible that these shocks actually occurred at shallower depths.

A relatively low level of activity is shown near the surface. A sys-

tematic error in depth determination could be responsible for this

effect however, computed depths for megaton-yield explosions, which

were all detonated near 1 km, were within a few tenths of a km of

the correct depth. Where reliable ocal depths have been determined

for natural earthquake activity in Nevada most depths range from 5 to

15 km (Ryall and Savage, 1969).

The depth distribution for the periods following the Benham, Purse,

Jorum and Handley explosions are shown in Figure 19b-e. A sharp reduc-

tion in activity below 5 km depth is shown for the periods following

Benham, Jorum and Handley. The period following Purse, in contrast,

shows a peak from 6 to 8 km. Most of these shocks occurred from 21 to

27 August in the area of activity about 7 km east-southeast of Benham

(Figure 14). As was mentioned earlier, the relationship of this

49


Figure 19.--Depth distribution of earthquakes listed in the Appendix with aquality rating of A or B which means that the focal depth is relativelywell determined. Periods covered by each part of the figure are: (a) wholeperiod, (b) Benham to Purse, (c Purse to Jorum, (d) Jorum to Handley, and

(e) Handley to the end.

activity to an explosion is unclear. The greater focal depths could

be an indication that the activity is natural.

The four time periods used in presenting the depth distribution

in Figure 19b-e are also used in summarizing the areal distribution

of earthquakes. For each earthquake located, the epicentral distance

was computed with respect to the explosion ground zero at the begin-

ning of the period. The distributions of tnese distances are shown

in Figure 20, parts b-. The cumulative distribution shown in part

of Figure 20 was formed by summing parts b-e.

The histograms in Figure 20 sow the general earthquake distri-

bution with respect to the explosions. The activity is not broadly

dispersed: 95% of the eicentral distances are less than 14 k

(Figure 20a). Forty-six earthquakes re located at distances greater

than 19 km. Of these, 3 occurred in the period following Benham, 1

after Purse, 11 ater Jorum, and 31 after Hndley. Their locations

are shown on the epicenter maps (Figures 1-18). Most of the more

distant shocks, particularly those following Handley, were associated

with the trend of activity hat developed about 22 km southeast of

the Benham site (Figures 17 and 18).

The distance distributions for the different explosions (Figure

20, parts b) show some variations. The activity following the

Benham ad andley explosions was concentrated several km from round

zero, whereas that following Jorum was ore closely associated with

the explosion site. The broader distribution for te period follow-

ing Purse merely reflects that Purse did not stimulate a high level

51

of activity, but that activity during the period was relatively

dispersed.

The number of earthquakes of a given magnitude or larger was

determined for the whole recording period, and for each of the four

intervals (Figure 21). The data above M - 2 for the whole period

define a straight line with a negative slope of approximately 1.4.

Data for the intervals show similar trends. Typically the slope of a

frequency-magnitude distribution, the "b value," s within a few

tenths of 1.0, except for swarms in volcanic areas which usually have

somewhat larger values, often above 2. The h-value of 1.4 found for

Pahute Mesa activity would be nearer l. if the M for the larger

earthquakes are underestimated,

In vw of the complications n relating magnitudes of small

earthquakes (2) to those of internediete-magnitude earthquakes (3-4),

it is not felt that the b-value of 1.4 significantly high for

example, if the mgnitudes of 3 1/2 were revised to 4 to bring them

into closer agreement with the USCGS magnitudes and smaller magnitudes

we e decreased by progressively maller amounts dn to about magnitude

2, then the resulting b-value would be near Such revisions cer-

tainly are not unreasonable. The main value of Figure 1 lies in

showing the similarity of the frequency-magnitude plots for the four

recording periods.

{COULD NOT BE CONVERTED T

OSEARCH

ABLE TEXT}

EARTHQUAKE DISTRIBUTION ANT AULTS

Most of the faults exposed at the surface of Pahute Mesa 3trike

north to about N 20 E. These faults are superposed on presumably

arcuate faults within celderas associated with old volcanic centers

now covered by younger volcanic rocks exposed at the surface of the

mesa. The Silent Canyon caldera is the principal caldera under ahute

Mesa and is outlined by the arcuate northeast trending dashed line on

Figure 22. The dshed lines that trend approximately east-west outline

part of the northern boundary of the Timber Mountain caldera complex,

which younger than, and includes, an unknown part of the Silent

Canyon caldera.

The relationship between these geologic features and he epicenter

pattern is shown in Fgure 2. The clear north-striking trend est and

southwest of Benham is in good alinement with a fault trend. The

northeast-striking trends in the epicenter pattern north of Benham are

subparallel with numerous relatively short fault segments. Much of the

Benham and Jorum aftershock activity occurred within the Slent Canyon

caldera, although the north-striking zone southwest of Benham extended

into the Timber Mountain caldera. A zone of activity southwest of

Benham lies along the Timber Mountain caldera boundary. Aftershocks

of Handley were mostly outside the Slent anyon caldera.

An attempt to associate the earthquakes with specific mapped

faults meets with only partial success. The relatively well-

delineated north-striking trend southwest of Benham is in good

Figure 22.--Map of all epicenters listed in the Appendix.

Mapped faults (solid lines , and caldera boundaries

(dashed lines) are from Orkild and others (1969).

Explosion locations are represented by large circles.

56

alinement with a fault to the south and with several shorter faults

within the epicenter zone. Similarly a fault rns into te cluster

of picenters west of Handley; however, the area of activity is

too wide to associate with a single fault. The northeast-striking

epicenter trends north of Benham, on te other hand, cannot be crrc-

lated with similarly striking mapped

The surficial faulting caused by eplosions in the Pahute Mesa

area has been summarized by R P. Snyder (written communication).

Fault movement from Benham extended to 5 km from grourd zero, with

displacements reaching vertical and 3C cm right-lateral. Most

of the breaks were along a 9 km segment of the fault passing about

km east of Benham. Displacement further north on this fault

bad previously been caused by Boxcar. Essentially no Benham after-

shocks can be correlated with this fault. North-striking ground

breakage 2 km in length was also observed 1/2 k west of Benham

scattered aftershock activity was observed in this vicinity, with

only a vague north-striking trend suggested the epicenter pattern.

North-striking cracks were found on the fault 5 km west Benham.

No fault displacement was observed in the main aftershock zone.

Surficial faulting from Purse extended to 1 l2 km. No correla-

tion with aftershocks is apparent.

Jorum caused fault isplacement to about 4 km from rouad zero.

Displacements eached 60 cm vertical with essentially no lateral.

displacement. Much of the movement occurred on the Boxcar fault which

passed about 1 km east of Jorum. Other movement was on faults up to

4 km west of Jorum. Again, there is no clear basis for associating

the faulting with aftershocks.

Faulting from Pipkin occurred to about 2 km. The major displace-

ment was on the Boxcar fault southwest of ground zero where 21 cm

vertical movement was observed. Essentially no earthquake activity

closely followed Pipkin.

Handley caused faulting out to 8 km from ground zero. Maximum

vertical displacement was 45 cm; 12 cm of right-lateral slip was

observed 2 km west of Handley. Most of the surface breaks were north-

striking, in the area east of Handley. Vertical displacement up to

10 cm was also observed on the fault 1 1/2 km west of Handley, in the

main aftershock area. This is the only such instance observed during

the period of this study.

Fault-plane solutions provide useful additional information about

the relationship between the earthquake activity and the faulting.

The results for Benham were presented earlier (Hamilton and Healy,

1969), but are shown again in Figure 23 together with solutions for

Jorum and Handley aftershocks. In the north-striking lineation west

and southwest of Benham, strike-slip faulting is indicated. Alorg

the epicenter trend the sense of movement is right lateral. North-

irest of Benham, dip-slip faulting on northeast-striking faults is

indicated. The fault-plane solutions for Jorum aftershocks were

similar, and confirm that Jorum caused a northeast extension of the

Benham aftershock zone. The main Handley aftershock zone, although

not contiguous with the northern Benham and the Jorum aftershock

58

Figure 23.--Fault-plane solutions for aftershocks of Benham,Jorum and Handley plotted on a geologic map by Orkild andothers (1969). Each circle represents an equal-area pro-jection of the lower focal hemisphere. Compressional anddilatational quadrants of first motion are shown by darkand light areas, respectively. Some of the fault-planesolutions are composite; i.e., they are based on first-motion data from more than one earthquake. They areadjacent to the numbers on the map which give the number

of earthquakes included.

59

zones, exhibited a similar dip-slip fault plane solution. The cluster

of aftershocks south-southwest of Handley, however, indicated strike-

slip faulting.

60

VERTICAL SECTIONS

The epicenters of earthquakes with relatively well-determined

focal depths are plotted in Figure 24 with a symbol indicating focal

depth to the nearest km. This figure was prepared to help choose lines

for vertical sections. Sections A-A' was selected approximately per-

pendicular to the fault that runs into the cluster of Handley after-

shocks. No clear epicenter trends are apparent in the cluster, but

most of the deeper shocks are near the cluster's center and vaguely

suggest a trend along the fault. Section B-B' includes the activity

near Jorum and is perpendicular to the northeast-striking trend pass-

ing southeast of Jorum. Section C-C' is perpendicular to the northeast-

striking epicenter trends northwest of Benham. Finally, section D-D'

is across the north-striking trend southwest of Benham, and includes

the activity along the Timber Mountain caldera boundary.

By comparing the epicenter map of Figure 24 with the vertical

sections in Figure 25 the distribution of the well-located earthquakes

with respect to the explosions can be seen. In section A-A', the main

zone of activity is centered about 3 km below and 1 km west of Handley.

No tabular or linear features are strongly suggested. Section B-B'

displays the activity clustered near Jorum, and the linear trend of

earthquakes passing about 3 km below and 3 km southeast of Jorum.

The earthquake distribution in section C-C' is relatively complex.

The lower boundary of activity exhibits a nrth-westerly dip. Most of

the earthquakes occurred from 0 to 4 km below Benham. Section D-D'

61


Figure 24.--Eoicenter map of earthquakes listed in the Appendix with

relatively well-determined focal depth. The number plotted at the

epicenter gives the focal depth to the nearest km up to 9 km; a

depth of 10 km is represented by A, 11 km by B, etc. Criteria in

choosing these earthquakes were ERH < 2.5 km, ERZ < 5.0 km, MD

< 0.2 sec, and NO > 7 stations, which are some of the criteria met

by Aand quality solutions. The lines f vertical section

(Figure 25) are indicated by A - A, 8 - B, C - C', and D -

Circles represent the explosion sites.

62


Figure 25.--Vertical sections along the lines indicated inFigure 24. Explosions are indicated by circles: in A - AHandley is shown, in B - B' Jorum, and n C - C', from left

to right, Purse, Benham and Pipkin.

63

shows well-defined, vertically-oriented, planar earthquake zone.

Along this zone right-lateral trike-slip fault movement indicated

by the fault-plane solutions.

64

1. Aftershock sequences were initiated by the underground nuclear

explosions Benham, Purse, Jorus and Handley. Their relative

effects are indicated below

Benham Purse Jorum Handley

Approximate duration ofaftershock sequence inJays (Figure 9): 70 10 20 60

Number of earthquakeswith anitude > 2.0(Appendix): 2012 24 159 231

Number of earthquake.listed in Appendix 3836 58 278 719

2. 942 of the earthquakes with well-determined focal depths occurred

shallower than 5 km.

3. 95 of the located earthquakes were within 14 km of ground zero

of the preceding explosion. There is no evidence for explosion-

stimulated earthquake activity expending utside the area of

Pahute Mesa.

4. Part of t Benham aftershock zone was active before detonation

of Benham This activity could be aftershocks of Boxcar.

5. Purse reactivated uch of the Benham aftershock zone.

6. Jorum caused growth of the Benham aftershock zone.

7. Pipkin hd essentially no effect on the seismicity, even though

it was larger in yield than Purse and situated near the areas of

earlier seismic activity.

5. Handley caused earthquakes in a new area, and stimulated activity

in areas affected by earlier explosions.

9. Early aftershock activity of Benham, Jorum and Handley was

predominantly near the shots (within about 2 k), with the more

distant activity beginning after cavity collapse.

Earthquake sequences occurred with epicenter distributions similar

in trend to some of those of the aftershocks, but that were not

clearly related to explosions either spatially or temporally.

l1. Earthquakes continued to be detected at a rate of about one a day

more than six months after an explosion had been detonated.

12. The spatial distribution of earthquakes appears to be largely

controlled by geologic structure.

13. The epicenter distribution can be associated with observed urfi-

cial faulting only for aftershocks of Handley.

14. Fault-plane olutions indicate predominant dip-slip movement in

the northern part of the Pahute Mesa area for aftershocks of

Benham Jorum and Handley. In the southern part, right-lateral

strike-slip movement was found for aftershocks of Benham and

Handley.

15. he frequency-magnitude relationships are similar for earthquakes

following Benham, Purse, Jorum and Handley.

16. Seven relatively small explosions in the southeastern part of the

area studied had no apparent effect on seismicity.

REFERENCES CITED

Aki, Kaiiti, 1969, Analysis of the seismic coda of local earthquakes

as scattered waves: Jour. Geophys. Research, v. 76, no. 2,

p. 615-631.

Basham, P. W., Weichert, D. H., and Anglin, F. M., 1970, An analysis

of the 'Benham' aftershock sequence using Canadian recordings:

Jour. Geophys. Research, v. 75, no. 5, p. 145-1556.

Bisztricsany, E., 1958, A new method for the determination of the

magnitude of earthquakes: Geofizikai Kozlemenyek, k. 7, sz. 2,

p. 69-96.

Boucher, Gary, Ryall, Alan, and Jones, A. ., 1969, Earthquakes

associated with underground nuclear explosions: Jour. Geo?hys.

Research, v. 74, no. 15, p. 3808-3820.

Eaton, J. P., O'Neill, M. E., and Murdock, J. N., 1970, Detailed

study of aftershocks of the 1966 Parkfield-Cholame, California,

earthquake: Seismol. Soc. America Bull., v. 60, no. 4, p. 1151-

1198.

Eaton, J. P., 1969, HYPOLAYR, a computer program for determining

hypocenters of local earthquakes in an earth consisting of

uniform flat layers over a half space: U.S. Geol. Survey open-

file rept., 106 p.

Evernden, J. F., 1967, Magnitude determination at regional and near

regional dstances in the United States: Seismol. Soc. America

Bull., v. 57, no. 4, p. 591-639.

67

Hamilton, R. M., and Healy, J. H., 1969, Aftershocks of the BENHAM

nuclear explosion: Seismol. Soc. America Bull., v. 59, no. 6,

p. 2271-2281.

Hamilton, R. M., MKeown, F. A., and Healy, J. H., 1969, Seismic

activity and faulting associated with a large underground nclear

explosion: Science, v. 166, no. 3905 p. 601-609.

Hamilton, R. M., Smith, B. E., Hall, J. C., and Healy, J. H., 169,

Summary of seismic activity in the Pahute Mesa area, Nevada Test

Site: December 8, 1968 - June 30, 1969, U.S. Geol. Survey rept.

USGS-474-58, 64 p.: available only from U.S. Dept. Commerce,

Natl. Tech. Inf. Service, Springfield, Va. 22151.

Healy, J. H., Hamilton, R. M., and Raleigh, C. B., 1970, Earthquakes

induced by fluid injection and explosion: Tectonophysics, v. 9,

p. 205-214.

Orkild, P. P., Sargent, K. A., and Snyder, R. P., 1969, Geologic map

of Pahute Mesa, Nevada rest Site and vicinity, Nye County,

Nevada: U.S. Geol. Survey, Misc. Geol. Inv. Map 1-567.

Prodehl, Claus, 1970, Seismic refraction study of crustal structure

in the western United States: Geol. Soc. America Bull., v. 81,

p. 2629-2646.

Richter, C. F., 1958, Elementary seismology: San Francisco and

London, W. H. Freeman, 768 p.

Ryall, Alan, and Savage, W. U., 1969, A comparison of seismological

effects for the Nevada underground test Boxcar with natural

68

earthquakes in the Nevada region: Jur. Geophys. Research, v. 74,

no. 17, p. 4281-4289.

Springer, D. L., and Kinnaman , R. L., 971, Seismic source summary for

U.S. underground nuclear explosions: Larence Radiazion Lab.

report UCRL - 73036, 29 p.

Stauder, W., 1971, Smaller aftershocks of the Benham nuclear explosion:

Seismol. Soc. America Bull., v. 61, no. 2, p. 417-428.

Tsmura, K., 1967, Determination of earthquake magnitude from total

duration of oscillation: Tokyo Univ. Earthquake Research Inst.

Bull., v. 45, p. 7-18.

69

APPENDIX

A chronological listing of the seismic events is contained in

this appendix. For each event the following data are given:

Origin time in Greenwich Civil Time: date, hour (R), minute (MN)

and second (SEC).

Epicenter in degrees and minutes of north latitude (LAT N) and

west longitude (LONG W). Poor convergence of the epicenter

solution is indicated by ""

DEPTH - depth of focus in km. Assumed depth is indicated by

MAG - earthquake magnitudes are (see discussion in text of

this report.); explosion magnitudes are from NOAA/NOS

reports.

NO - number of stations used in locating earthquake.

GAP largest azimuthal separation in degrees between stations.

DMIN epicentral distance in km to the nearest station.

ERT standard error of the origin time in seconds.

ERH standard error of the epicenter in km.where DZ and DY are the standard errors in latitude and

longitude, respectively, of the epicenter.

ERZ - standard error of the depth in km.

MD - mean deviation of the time residuals. where

R is the observed seismic wave arrival time less the computed

time at the ith station.

70

solution quality of the hypocenter. This measure is intended

to indicate the general reliability of each solution:

Q Epicenter Focal Depth

A excellent good

B good fair

C fair poor

D poor poor

Q is based on both the nature of the station distribution

with respect to the earthquake and the statistical measures

of the solution. These two factors are each rated inde-

pendently according to the following scheme:

Station Distribution

NO GAP DMIN

A > 8 <120 < DEPTH or 5 km

> 6 <150 < 2 x DEPTH or 10 km

C > 6 < 225 < 50 km

> 4 < 180°

D Others

Statistical Measures

ERH(km) ERZ(km) MD(sec) RMAX(sec)*

A < 1.0 < 2.0 < 0.10 < 0.25

B < 2.5 <5.0 < 0.20 < 0.50

C < 5.0 < 0.30 < 0.75

D Others

RMAX is the maximum residual

Q is taken as the average of the ratings from the two schemes,

i.e., an A and a C yield a B and two B's yield a B. When the

two ratings are only one level apart the lower one is used, i.e.,

an A and a yield a B.

APPENDIX: PAHUTE MESA SEISMICITY{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX: PANUTE

APPENDIX PAHUTE MESA SEISMICITY (CONTINUED){COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PAHUTE MESA SEISMICITY{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PAHUTE MESA SEISMICITY CONTINUED

APPENDIX: PAHUTE MESA SEISMICITY (CONTINUED)

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED)

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED){COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX: PAHUTE MESA SEISMICITY (CONTINUED

APPENDIX PAHUTE MESA SEISMICITY (CONTINUED{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PHUTE MESA SEISMICITY CONTINUED)

APPENDIX PAHUTE MESA SEISMICITY COSNTINUED

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED)

APPENDIX, PAHUTE MESA SEISMICITY CONTINUED)

APPENDIX PAHUTE MESA SEISMICITY CONTINUED{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PAHUTE MESA SEISMICITY CONTINUED)

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED

APPENDIX PAHUTE MESA SEISICITY CONTINUED

APPENDIX PANUTE MESA SEISMICITY CONTINUED

APPENDIX PAMUTE MESA SEISMICITY CONTINUED

APPENDIX: PAHUTE MESA SEISMICITY (CONTINUED){COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PAMUTE MESA SEISMICITY CONTINUED

APPENDIX: PAHUTE MESA SEISMICITY (CONTINUED

APPENDIX PAHUTE MESA SEISMICITY CONTINUED){COULD NOT BE CONVERTED TO SEARCHALE TEXT}

APPENDIX: PAHUTE ESA SEISMICITY CONTINUED

{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}APPENDIX PAHUTE MESA SEISMICITY CONTINUED

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

APPENDIX PAHUTE MESA SEISMIClTY CONTINUED

APPENDIX: PUTE MESA SEISMICITY CONTINUED

APPENDIX PAHUTE ESA SEISMICITY CONTINUED

APPENDIX: PAHUTE MESA SEISMICITY CONTINUEDI

APPENDIX: PAHUTE MESA SEISMICITY CONTINUED{COULD NOT BE CONVERTED TO SEARCHABLE TEXT}

USGS-474-138, 'Seismicity of the Pahute Mesa Area, Nevada ...

Documents

Transcript of USGS-474-138, 'Seismicity of the Pahute Mesa Area, Nevada ...